Aggregate is used in many industries for various applications. Typically the aggregate is stored without regard to its cleanliness. Prior to its use, the aggregate must be cleaned of all the accumulated rock dust and other contaminents. This is readily accomplished by spraying the aggregate with water. In order to reduce the cost associated with cleaning aggregate, the waste water is recycled. In addition, water must be clear and non-turbid before discharge into any waterway. The main goal of recycling is to remove the solids suspended in the waste water.
Waste water settling technology is used in other mining industries and various types of industrial applications where process water is used ranging from food processing to gas manufacture to steel making. The waste water settling apparatus and method are equally applicable to these industries as well as municipal waste water treatment.
Conventional waste water settling processes are used to remove the solid content from the water so the water can be reused or discharged safely and removed solids can be disposed of properly. Conventional devices or methods use a clarifier in combination with a belt press or filter to remove the solids from the water or a settling pond.
A clarifier is a large holding tank. Waste water is fed into the tank and a chemical flocculent is added. The flocculent causes the solid particles to adhere to one another forming larger particles. These larger particles settle on the bottom of the clarifier. Water above the settled particles is discharged or recycled. Solids accumulated in the bottom of the clarifier are fed to a belt press or filter.
Belt presses include one or more endless belts fed between pairs of rollers. Sludge from a clarifier is fed between the belts. The belts carry the sludge through pairs of rollers. The rollers extract water and collect solids between the belts. Filters work in much the same way as a belt press. Water is passed though the filter and solids are captured for disposal.
Both conventional belt presses and filters are costly to operate and require frequent maintenance. The filters and the belts in the press need to be replaced periodically. Also, the cost of the belt presses and filter assemblies are extremely high. Belt presses frequently require a full time operator because of the frequent adjustments and high maintenance requirements. Filters require periodic back washes to clean the filters resulting in noncontinuous operation.
Another approach for settling solids out of waste water involves the use of large settling ponds. Settling ponds usually cover a large area, thus requiring a significant cost in land. Typical settling ponds may be 10 to 20 acres in size.
Waste water is fed into the pond through an inlet located at one end and recyclable water is removed from a discharge outlet at the other end. The water fed into the pond slowly flows from the intake to the outlet. As the water flows to the outlet, the difference in the specific gravity between the water and the suspended solids causes the suspended solids to settle gradually to the bottom of the pond. Larger particles settle rapidly while the smaller particles take longer to settle to the bottom of the pond. Due to the difference in settling times as the water gradually flows from the inlet to the outlet, the composition of particles along the bottom of the pond is non-uniform. Larger particles settle out quickly near the intake, while smaller particles settle more slowly with the smallest settling near the outlet. Accumulated solids are typically dredged from the bottom of the pond using a drag line. This method of recovering the solids leaves the solids with a high moisture content. High moisture content increases the weight of the solids requiring high transportation costs for disposal. In addition, the non-uniform composition of the settled solids decreases their value for other applications.